1. Field of the Invention
The present invention relates to a track-follow control device used in a disc system using a floppy disc, a hard disc, a digital video disc, a compact disc or an optical disc.
2. Description of the Related Art
A disc such as a floppy disc or hard disc is divided into concentric tracks, and each track is physically or logically divided into sectors or the like. Information is read or written by using an actuator to move a recording/reproducing head to the position of a target track on the divided regions.
A shift in the center of a motor for rotating a disc recording medium, a shift in chucking due to a shift in the center of the recording medium, and deformation of the recording medium due to a temperature increase cause eccentricity.
When the information is read or written, the position of the head is determined by the actuator. At this time, the eccentricity shifts the position of the head from the target track, which thus generates a phase shift as represented by A and an amplitude shift as represented by B. Accordingly, when the position of the head is determined by the actuator, the actuator must be controlled by its control system so that information reading or writing is not affected.
A conventional track-follow control device controls the phase shift and the amplitude shift by means of repeat learning control. In FIG. 3, an example of the conventional track-follow control device using repeat learning control is shown. As shown in FIG. 3, the conventional track-follow control device includes a main-controller 60 for performing velocity compensation, positional compensation and integral compensation, a sub-controller 70 for performing repeat learning control, and an object 80 to be controlled such as an actuator. The sub-controller 70 includes a delay circuit 71 and a band limit filter 72.
Concerning the conventional track-follow control device shown in FIG. 3, the principle of the repeat learning control by the sub-controller 70 will be described below using FIG. 4 in which an example of a repeat-learning-control compensation signal is shown. In FIG. 4, square points represent a positional error signal showing the positional error between the target track and the head, and lozenge points represent a compensation signal caused by the repeat learning.
Referring to a deviation of zero degrees, the sub-controller 70 performs the repeat learning control by adding the present positional deviation to the total of previous positional deviations at zero degrees to form the compensation signal. Specifically, by letting an initial total positional deviation be zero, a positional deviation represented by a1 is added to the initial total positional deviation of zero to form a compensation signal at next zero degrees denoted by b1. Similarly, a positional deviation represented by a2 at zero degrees (b1) is added to the total of previous positional deviations as represented by C1 to form a compensation signal at zero degrees represented by b2. The above-described repeat learning is performed, whereby the compensation signals accumulate until the deviation at zero degrees is an average of zero, and the accumulation is output.
The motion of the actuator as the object 80 to be controlled has physical limitation. Thus, compensation up to high frequencies cannot be performed. Accordingly, the integration result by the repeat learning diverges to infinity so that control up to deviations of zeroes at all frequencies is performed. This divergence is prevented by the band limit filter 72.
According to the conventional track-follow control device, if the band limitation by the band limit filter 72 fails to be set to an optimum range, an appropriate control cannot be performed.
In addition, in digital processing, the integrator of the delay circuit 71 requires compensation-signal data on sampling points for one cycle. Thus, its memory (random access memory) for holding data must have a capacity in accordance with the number of the sampling points.